1. Field of the Invention
The present invention relates to an aromatic polyimide laminate and to a process for preparing the same. More particularly, the present invention relates to an aromatic polyimide laminate, having a thickness of 50 microns or more, comprising multiple aromatic polyimide film layers superimposed on and directly bonded to each other without using an adhesive. The polyimide laminates have excellent heat resistance, durability and adhesive bonding properties and can be used in various applications such as punched gaskets and washers, speaker coils, shim stock and circuit boards.
2. Description of the Prior Art
Commercial applications exist which require polyimide laminate structures ranging from 125 microns to 1500 microns or more in thickness. However, current aromatic polyimide films having a thickness of from about 25 microns to 125 microns are conventionally produced by casting a solution of a poly(amic acid) precursor of the polyimide in an organic solvent onto the surface of a support and then solidifying the thin solution layer to provide the polyimide film. This conventional method has disadvantages, since the polyimide polymer has poor solubility in the organic solvent and the resulting solution has high viscosity, thereby making thick film-formation difficult.
Accordingly, using the conventional method, it is difficult to produce a polyimide film having a thickness of, for example, 125 microns or more having satisfactory quality and at high reproducibility.
Furthermore, it is well-known that almost all aromatic polyimide polymers are non-fusible. Therefore, direct bonding of conventional aromatic polyimide films to each other to form thick laminates without using an adhesive or a bonding film, e.g. a fluorine-containing thermoplastic film, is very difficult.
Moreover, even if the aromatic polyimide films can be bonded to each other using an adhesive, the resultant polyimide laminate contains an adhesive layer which usually has a much poorer heat resistance and durability than those of the aromatic polyimide film and, thus, exhibits decreased heat resistance and durability.
U.S. Pat. No. 5,227,244, issued to Milligan on Jul. 13, 1993, discloses a poly(amic acid) film coated with an organic solvent solution of a metal salt and heating the coated film to convert the poly(amic acid) to the polyimide. A laminate of the polyimide film with a copper foil exhibits improved adhesion when bonded together using an acrylic adhesive.
U.S. Pat. No. 5,218,034, issued to Milligan et al on Jun. 8, 1993, discloses adhesively bonded polyimide film copper laminates wherein the polyimide film contains from 0.02 to 1% by weight of tin introduced as a tin (II) or tin (IV) salt.
U.S. Pat. No. 5,272,194, issued to Arduengo et al on Dec. 21, 1993, discloses a polyimide film containing from 0.02 to 1% by weight of metal from an organometallic compound wherein the metal is tin, bismuth or antimony, which has improved adhesion when bonded to a metal foil through a heat-resistant adhesive.
In the aforementioned patents, the metal containing or metal coated polyimide film is directly bonded to a metallic substrate through an adhesive. There is no disclosure of adhesiveless polyimide laminates having thickness of 50 microns or more formed by bonding multiple layers of polyimide films at high temperature and pressure.
Accordingly, there is a need for an aromatic polyimide laminate having a relatively large thickness of 50 microns or more and which does not use an adhesive bonding agent.